As a display element provided in a display device, there have hitherto been an electrooptical element whose luminance is controlled by an applied voltage, and an electrooptical element whose luminance is controlled by a flowing current. Examples of the electrooptical element whose luminance is controlled by an applied voltage include a liquid crystal display element. Meanwhile, examples of the electrooptical element whose luminance is controlled by a flowing current include an organic EL element. The organic EL element is also called an OLED (Organic Light-Emitting Diode). An organic EL display device that uses the organic EL element being a spontaneous electrooptical element can be easily reduced in thickness and power consumption and increased in luminance as compared to the liquid crystal display device that requires a backlight, a color filter and the like. Hence in recent years, development of the organic EL display device has been actively advanced.
As drive systems for the organic EL display device, a passive matrix system (also called simple matrix system) and an active matrix system are known. As for an organic EL display device employing the passive matrix system, its structure is simple, but a large size and high definition are difficult to achieve. In contrast, as for an organic EL display device employing the active matrix system (hereinafter referred to as an “active matrix-type organic EL display device”), a large size and high definition can be easily realized as compared to the organic EL display device employing the passive matrix system.
In the active matrix-type organic EL display device, a plurality of pixel circuits are formed in a matrix form. The pixel circuit of the active matrix-type organic EL display device typically includes an input transistor for selecting a pixel and a drive transistor for controlling supply of a current to the organic EL element. It is to be noted that in the following, a current that flows from the drive transistor to the organic EL element may be referred to as a “drive current”.
FIG. 32 is a circuit diagram showing a configuration of a conventional general pixel circuit 91. This pixel circuit 91 is provided corresponding to each of intersections of a plurality of data lines S and a plurality of scanning lines G which are disposed in a display portion. As shown in FIG. 32, this pixel circuit 91 is provided with two transistors T1 and T2, one capacitor Cst, and one organic EL element OLED. The transistor T1 is an input transistor, and the transistor T2 is a drive transistor.
The transistor T1 is provided between the data signal line S and a gate terminal of the transistor T2. As for the transistor T1, agate terminal is connected to the scanning line G, and a source terminal is connected to the data signal line S. The transistor T2 is provided in series with the organic EL element OLED. As for the transistor T2, a drain terminal is connected to a power supply line that supplies a high-level power supply voltage ELVDD, and a source terminal is connected to an anode terminal of the organic EL element OLED. It should be noted that, the power supply line that supplies the high-level power supply voltage ELVDD is referred to as a “high-level power supply line” in the following, and the high-level power supply line is added with the same symbol ELVDD as that of the high-level power supply voltage. As for the capacitor Cst, one end is connected to the gate terminal of the transistor T2, and the other end is connected to the source terminal of the transistor T2. A cathode terminal of the organic EL element OLED is connected to a power supply line that supplies a low-level power supply voltage ELVSS. It should be noted that, the power supply line that supplies the low-level power supply voltage ELVSS is referred to as a “low-level power supply line” in the following, and the low-level power supply line is added with the same symbol ELVSS as that of the low-level power supply voltage. Further, here, a contact point of the gate terminal of the transistor T2, the one end of the capacitor Cst, and the drain terminal of the transistor T1 is referred to as a “gate node VG” for the sake of convenience. It is to be noted that, although one having a higher potential between a drain and a source is generally called a drain, in descriptions of the present specification, one is defined as a drain and the other is defined as a source, and hence a source potential may become higher than a drain potential.
FIG. 33 is a timing chart for explaining an operation of the pixel circuit 91 shown in FIG. 32. Before time t1, the scanning line G is in a non-selected state. Therefore, before the time t1, the transistor T1 is in an off state, and a potential of the gate node VG is held at an initialization level (e.g., a level in accordance with writing in the last frame). At the time t1, the scanning line G comes into a selected state and the transistor T1 is turned on. Thereby, a data voltage Vdata corresponding to a luminance of a pixel (sub-pixel) formed by this pixel circuit 91 is supplied to the gate node VG via the data signal line S and the transistor T1. Thereafter, in a period till time t2, the potential of the gate node VG changes in accordance with the data voltage Vdata. At this time, the capacitor Cst is charged with a gate-source voltage Vgs which is a difference between the potential of the gate node VG and a source potential of the transistor T2. At the time t2, the scanning line G comes into the non-selected state. Thereby, the transistor T1 is turned off and the gate-source voltage Vgs held by the capacitor Cst is determined. The transistor T2 supplies a drive current to the organic EL element OLED in accordance with the gate-source voltage Vgs held by the capacitor Cst. As a result, the organic EL element OLED emits light with a luminance in accordance with the drive current.
Incidentally, in the organic EL display device, a thin film transistor (TFT) is typically employed as the drive transistor. However, regarding the thin film transistor, variations in threshold voltage tend to occur. When variations in threshold voltage occur in the drive transistor provided in the display portion, variations in luminance occur, and the display quality thus deteriorates. Accordingly, a technique of suppressing deterioration in display quality in the organic EL display device has hitherto been proposed. For example, Japanese Patent Application Laid-Open No. 2005-31630 discloses a technique of compensating for variations in threshold voltage of a drive transistor. Further, Japanese Patent Application Laid-Open No. 2003-195810 and Japanese Patent Application Laid-Open No. 2007-128103 each discloses a technique of making constant a current flowing from a pixel circuit to an organic EL element OLED. Moreover, Japanese Patent Application Laid-Open No. 2007-233326 discloses a technique of displaying an image with a uniform luminance regardless of electron mobility and a threshold voltage of a drive transistor.
According to the foregoing prior arts, even when variations in threshold voltage occur in the drive transistor provided in the display portion, it is possible to supply a constant current to the organic EL element (light-emitting element) in accordance with a desired luminance (target luminance). However, as for the organic EL element, current efficiency decreases with the lapse of time. That is, even when a constant current is supplied to the organic EL element, the luminance gradually decreases with the lapse of time. This results in occurrence of burning.
Thus, unless compensation is performed on degradation of the drive transistor and degradation of the organic EL element, current decrease due to the degradation of the drive transistor occurs and luminance decrease due to the degradation of the organic EL element occurs as shown in FIG. 34. Further, even when compensation is performed on the degradation of the drive transistor, luminance decrease due to the degradation of the organic EL element occurs with the lapse of time as shown in FIG. 35. Accordingly, Japanese Translation of PCT International Application Publication No. 2008-523448 discloses a technique of correcting data based on a characteristic of the organic EL element OLED in addition to the technique of correcting data based on a characteristic of the drive transistor.